1. Field of the Invention
The invention relates to channel estimation, and in particular to channel estimation in a spread spectrum receiver.
2. Description of the Related Art
A spread spectrum system combats multipath interference by multiplying narrowband data by a wideband pseudo noise (PN) code in a spread spectrum transmitter to produce a wideband transmitted signal with nearly the spectrum of the PN code. Examples of spread spectrum systems can be found in Code Division Multiple Access (CDMA) and Wideband CDMA (WCDMA) systems conforming to the UMTS/IMT-2000 (Universal Mobile Telecommunications System/International Mobile Telecommunications-2000) specifications. Typically, spread spectrum systems such as WCDMA systems employ an orthogonal spreading code spreading a data symbol over a desirable spectrum and pseudo noise (PN) scrambling codes separating different cells in downlink transmission and user in uplink transmission. The spreading codes, also known as orthogonal channelization codes, transform a data symbol into a number of chips as the spread-spectrum signal prior to data transmission. Spread spectrum receivers such as RAKE receivers, frequently used in CDMA systems, descramble, and/or despread the received spread spectrum, reducing signal multipath reception to improve reception quality. Spreading and despreading codes may be Gold codes, Walsh codes, orthogonal variable spreading factor (OVSF) channelization codes and/or equivalent code sequences.
A pilot channel is typically used for channel estimation to control various parameters of a received spread spectrum signal at reception, compensating various channel conditions in a particular transmission channel. In wireless communications, channel conditions vary over both time and frequency, thus wireless communications systems, such as spread spectrum communication systems, must estimate and compensate for varying channel conditions to the received spread spectrum signal. In WCDMA downlink transmission, the pilot channel is referred to as common pilot channel (CPICH). Since pilot symbols in CPICH are transmitted with a higher power then other dedicated traffic channels, the result is a better reception, whereby the channel condition can be accurately estimated.
There is a need for a spread spectrum receiver and a method thereof accurately estimating channel conditions for spread spectrum signals.